FIG. 9 shows an electrically movable wheel hub unit 2 for an electrically assisted bicycle.
The electrically movable wheel hub unit (hereinafter referred to as the “hub unit”) has a motor 9 comprising a stator 3 and a rotor 4, a hub 7 enclosing the motor 9 therein and rotatingly drivable at a reduced speed by the motor 9 and a planetary gear reduction mechanism 6, and fixing support shafts 57, 57a disposed on the axis of rotation of the hub 7 fixedly relative to a motor housing 5 and projecting outward from the hub 7.
The stator 3 comprises a stack 31 of metal plates and windings 32 provided on the metal plate stack 3.
The motor housing 5 comprises a cylindrical wall 8 and a pair of end plates 51, 52 closing opposite end openings of the wall 8. The end plates 51, 52 are fastened to the cylindrical wall 8 with bolts 53b, 53b. 
The metal plate stack 31 of the stator 3 is fastened to one of the end plates, 51, with a plurality of bolts 53d extending through the outer peripheral portion of the stack.
The fixing support shafts 57, 57a extending through respective opposite ends of the hub 7 are fixed to the front or rear fork of the bicycle, and the spokes (not shown) of the wheel are fitted into spoke attaching holes 74a in circumferential walls 74, 74 provided around the outer periphery of the hub 7.
When windings 32 of the stator 3 are energized, the rotor 4 rotates. The rotation of the rotor 4 is transmitted to the hub 7 upon a speed reduction by the planetary gear reduction mechanism 6 to rotate the wheel of the bicycle.
In the case where a load not smaller than a specified value acts on the pedals of the bicycle when they are stepped on, the windings 32 of the stator 3 are energized, permitting the rider to run with ease even on slopes.
See, for example, JP-A No. 2002-514550.
The energization of the windings results in heat generation.
If the windings 32 are overheated, the rotor 4 can not be rotated with high torque, hence a need for a contrivance for releasing the heat of the windings 32.
However, since the stator 3 is enclosed in the motor housing 5 which is closed, the stator 3 is unable to radiate heat effectively.
For this reason, it is practice to enclose a cooling liquid within the hub 7 to cool the motor housing 5 from outside and suppress the heat generation of the windings 32.
When the cooling liquid is enclosed in the hub 7, means or measure is needed for preventing leakage of the liquid to result in a corresponding increase in cost. The cooling liquid used correspondingly increases the weight of the hub unit.
The present invention provides a hub unit of the type described wherein the outer peripheral surface of the stator 3 is left exposed to face the inner surface of the hub 7, with the cylindrical wall 8, part of the motor housing 5, dispensed with so as to cause the stator 3 to release heat more effectively for the prevention of overheating. The invention also provides a vehicle comprising the hub unit.